ICS201 Lecture 17 : Applets

1. King Fahd University of Petroleum & Minerals College of Computer Science & Engineering Information & Computer Science Department ICS201 Lecture 17 : Applets

2. Applets: Introduction • Java programs are divided into two main categories, applets and applications • An applicationis an ordinary Java program • An applet is a kind of Java program that can be run across the Internet

3. Programming Applets • The word applet is meant to suggest a small application • Applets were intended to be small programs run over the Internet • However, there are no size constraints on applets • Applets can be viewed over the Internet, or without any connection to the internet • An applet is similar to a Swing GUI • In fact, almost all of the Swing techniques can be used in applets

4. Defining an Applet • An applet class is normally defined as a derived class of the class JApplet • The class JApplet is in the package javax.swing • There is also an older class, Applet, which has been superseded by the JApplet class

5. Applets in the Class Hierarchy

6. Designing an Applet • An applet class can be designed as a derived class of JApplet in much the same way that regular Swing GUIs are defined as derived classes of JFrame • However, an applet normally defines no constructors • The method init performs the initializations that would be performed in a constructor for a regular Swing GUI

7. Designing an Applet • Components can be added to an applet in the same way that a component is added to a JFrame • The method add is used to add components to an applet in the same way that components are added to a JFrame

8. An Applet (Part 1 of 2)

9. An Applet (Part 2 of 2)

10. How Applets Differ from Swing GUIs • Some of the items included in a Swing GUI are not included in an applet • Applets do not contain a main or setVisible method • Applets are displayed automatically by a Web page or an applet viewer • Applets do not have titles • Therefore, they do not use the setTitle method • They are normally embedded in an HTML document, and the HTML document can add any desired title

11. How Applets Differ from Swing GUIs • Applets do not use the setSize method • The HTML document takes care of sizing the applet • Applets do not have a close-window button • Therefore, they do not have a setDefaultCloseOperation method • When the HTML document containing the applet is closed, then the applet is automatically closed

12. Running an Applet • An applet class is compiled in the same way as any other Java class • However, an applet is run differently from other Java programs • The normal way to run an applet is to embed it in an HTML document • The applet is then run and viewed through a Web browser

13. Running an Applet • An applet can also be viewed using an applet viewer • An applet viewer is a program designed to run an applet as a stand-alone program • The Java appletviewer can be used to run an applet: appletviewer FirstApplet.html • It may be necessary, however, to create the HTML document, and place the applet in it

14. Applet Examples • http://www.nku.edu/~foxr/Camp/appletexamples.html

15. Menus in a JApplet • Menus are constructed and added to a JApplet as they are for a JFrame • JApplet has a method named setJMenuBar that behaves the same as the setJMenuBar method of a JFrame • JApplet can also have menu bars added to a JApplet or to a panel that is part of the JApplet using the add method

16. Tip: Converting a Swing Application to an Applet • The fastest and easiest way to explain how to define an applet, is to explain how to modify a Swing GUI to transform it into an applet • Derive the class from the class JApplet instead of from the class Jframe • Remove the main method • Replace the constructor with a no-parameter method named init • The body of the init method can be the same as the body of the deleted constructor, but with some items removed • Delete any invocation of super • Delete any method invocations that program the close-window button of a windowing GUI • Delete any invocation of setTitle • Delete any invocation of setSize • The following applet was generated in this way

17. An Applet Calculator (Part 1 of 9)

18. An Applet Calculator (Part 2 of 9)

19. An Applet Calculator (Part 3 of 9)

20. An Applet Calculator (Part 4 of 9)

21. An Applet Calculator (Part 5 of 9)

22. An Applet Calculator (Part 6 of 9)

23. An Applet Calculator (Part 7 of 9)

24. An Applet Calculator (Part 8 of 9)

25. An Applet Calculator (Part 9 of 9)

26. Inserting an Applet in an HTML Document • An applet can be placed in an HTML document with an applet tag: <applet code="PathToApplet" width=Number1 height=Number2> </applet> • If given a .class file name only, then the HTML file and the applet file must be in the same directory • The PathToApplet can be a full or relative path name

27. Inserting an Applet in an HTML Document • Note that the name of the .class file, not the .java file, is given • Note also that the width and height of the applet is given in this command, and not within the applet class definition • The width and height are in pixels • The following code, when placed in an HTML document, will display the calculator applet in a browser as shown <applet code="AppletCalculator.class" width=400 height=300> </applet>

28. An Applet in an HTML Document <html> <head> <title> Vampire Control </title> </head> . . . <applet code="AppletCalculator.class" width=400 height=300> </applet> . . . </html>

29. Browser View

30. Applets and Security • An applet can be a program, written by someone else, that runs on your computer • Whenever someone else's program runs on your computer, there are security questions you should ask: • Will it read information from your files? • Will it corrupt your operating system? Applets are designed so that they cannot do any of these things (at least easily)

31. Applet Examples • http://www-math.mit.edu/daimp/

32. The end